Conventionally, in an image forming apparatus that employs an electrophotographic printing method, a charging device that employs a corona discharge system has been used in, for example, charging means for charging a photoreceptor that is an image bearing member for bearing an electrostatic latent image, transfer means for transferring a toner image formed on the photoreceptor to recording paper that is a transfer receiving material and a recording medium via a transfer belt that is a transfer receiving material and an intermediate transfer body, and separation means for separating the recording paper which electrostatically comes into contact with the photoreceptor or the like.
Patent Document 1 (Japanese Unexamined Patent Publication No. 11946/1994 (published on Jan. 21, 1994)) discloses such a charging device employing a corona charging system. This charging device includes: a shield case having an opening section which faces a charge receiving material such as a photoreceptor and a transfer belt; and a discharge electrode whose discharging surface has a line shape, a saw-tooth shape, or a needle shape and which is provided in a tensioned state in the shield case. The charging device disclosed in Patent Document 1 is (a) a corotron that applies a high voltage to the discharge electrode so as to generate corona discharge, thereby uniformly charging a charge receiving material, or (b) a scorotron that applies a desired voltage to a grid electrode provided between a discharge electrode and a charge receiving material, thereby uniformly charging the charge receiving material.
FIG. 12 is a drawing explaining a charging mechanism in a charging device employing the corona discharge system. By applying a high voltage across a discharging electrode 71 with small curvature radius and a grid electrode 72, an uneven electric field is generated between the two electrodes. Consequently, local ionization occurs due to a high electric field in the vicinity of the discharging electrode 71, electrons are discharged toward the charge receiving material 11 (in a direction of an arrow D in FIG. 12) (discharge due to electron avalanche), and a toner 12 on the charge receiving material 11 is charged. The grid electrode 72 is used to control the amount of electrons moving toward the charge receiving material 11. Electrons are also discharged to the grid electrode 72.
The charging device employing the corona discharge system is used in a pre-transfer charging device for charging a toner image that has not been transferred yet to a transfer medium such as an intermediate transfer body and a recording paper. Examples of such a charging device are disclosed in Japanese Unexamined Patent Publication No. 274892/1998 (Tokukaihei 10-274892) (published on Oct. 13, 1998) (Patent Document 2) and Japanese Unexamined Patent Publication No. 69860/2004 (Tokukai 2004-69860) (published on Mar. 4, 2004) (Patent Document 3). According to techniques as disclosed in Patent Documents 2 and 3, even if a charge amount is not uniform in the toner image formed on an image bearing member, the charge amount of the toner image is uniform before the toner image is transferred. Therefore, it becomes possible to suppress a decrease in a transfer margin at the time of transferring a toner image, and also to stably transfer the toner image to a transfer medium.
However, the conventional charging device described above has a plurality of problems. The first problem concerns a space in which the charging device is provided. The charging device employing the corona discharging system requires not only the discharge electrode 71 but also the shield case, the grid electrode 72, and the like. Further, it is necessary to ensure a considerably large distance (e.g. 10 mm or so) between the discharge electrode 71 and the charge receiving material 11. As a result, a large space is necessary for providing the charging device. In an image forming apparatus, a photoreceptor, developing means for supplying a toner to an electrostatic latent image on the photoreceptor to form a toner image on the photoreceptor, first transfer means for transferring the toner image on the photoreceptor to a transfer belt, recording transfer means for transferring the toner image on the transfer belt to recording paper etc. are provided around the charging device. Consequently, a space for the charging device is small. Therefore, in the conventional charging device employing the corona discharge system, it is difficult to provide the charging device employing the corona discharge system that requires a comparatively large space.
The second problem concerns discharge products that are generated when the charging device charges the charge receiving material 11. As illustrated in FIG. 12, the charging device employing the corona discharge system generates a large amount of discharge products such as ozone (O3) and nitrogen oxide (NOx). Specifically, due to an energy derived from discharge of electrons from the charging device, nitrogen molecules (N2) in the air are separated into nitrogen atoms (N), and the nitrogen atoms bind to oxygen molecules (O2) to form nitrogen oxides (nitrogen dioxide: NO2). Similarly, oxygen molecules (O2) in the air are separated into oxygen atoms (O), and the oxygen atoms bind to oxygen molecules (O2) to form ozone (O3). Generation of a large amount of ozone causes (i) ozone smell, (ii) a harmful influence on a human body, (iii) deterioration of members due to strong oxidation power, and the like. Further, when nitrogen oxide is generated, nitrogen oxide as ammonium salt (ammonium nitrate) adheres to the photoreceptor. This causes a defect in an image. Especially, usage of an organic photoconductor (OPC) as a photoreceptor tends to cause a defect in an image, for example, a white spot or an image deletion because of ozone, NOx or the like.
The third problem concerns a corona wind generated when the charging device charges the charge receiving material 11. The corona wind is generated from the discharging electrode 71 toward the charge receiving material 11 due to the flow of electrons caused by corona discharge. When the charging device employing the corona discharge system is used as a pre-transfer charging device, the corona wind disturbs a toner image formed on the charge receiving material 11.
As a charging device capable of reducing generation of discharge products, there is proposed a charging device employing a contact electrification system in which a conductive roller or a conductive brush touches a charge receiving material in order to charge the charge receiving material. However, since the conductive roller or the conductive brush touches the charge receiving material in order to charge it in the charging device employing the contact electrification system, it is difficult to charge the charge receiving material without disturbing a toner image formed on the charge receiving material. Therefore, the charging device employing the contact electrification system is not appropriate for a pre-transfer charging device.
Japanese Unexamined Patent Publication No. 160711/1996 (Tokukaihei 8-160711) (published on Jun. 21, 1996) (Patent Document 4) discloses a charging device employing a corona discharge system, capable of reducing generation of discharge products. The charging device disclosed in Patent Document 4 includes: a plurality of discharge electrodes arranged at a substantially equal pitch in a predetermined axial direction; a high voltage power source for applying, to the discharge electrodes, a voltage equal to or higher than a predetermined voltage for starting discharge; a resistor provided between an output electrode of the high voltage power source and the discharge electrodes; a grid electrode provided in the vicinity of the discharge electrodes and between the discharge electrodes and the charge receiving material; and a grid power source for applying a predetermined grid voltage to the grid electrode. A gap between the discharge electrodes and the grid electrode is set to be 4 mm or less. Thus, by reducing the gap between the discharge electrodes and the grid electrode, it is possible to reduce a discharge current, thereby reducing generation of the discharge products.
However, the charging device disclosed in Patent Document 4 cannot sufficiently reduce generation of discharge products, and approximately 0.3 ppm of ozone is still generated. Further, since the charging device disclosed in Patent Document 4 has a small gap between the discharge electrodes and the grid electrode, foreign matters such as discharge products, a toner, and paper powder derived from recording paper that is a transfer receiving material tend to attach to the discharge electrodes. Since a discharge surface of the discharge electrode employing the corona discharge system has a complex shape such as a needle shape, the foreign matters attached to the discharge electrode are difficult to be removed (cleaned). Further, a tip of the discharge electrode is likely to be abraded/deteriorated due to discharge energy, which causes the discharge electrode to discharge unstably. Moreover, because a gap between the discharge electrodes and the charge receiving material is narrow, non-uniformity in charging easily occurs in a long direction (axial direction in which a plurality of discharge electrodes are aligned) due to the pitch of a plurality of the discharge electrodes. Here, a shorter pitch of the discharge electrodes may improve the non-uniformity in charging. However, this increases the number of the discharge electrodes, which increases production cost.
In view of such problems, Japanese Unexamined Patent Publication No. 173744/2000 (Tokukai 2000-173744) (published on Jun. 23, 2000) (Patent Document 5), Japanese Unexamined Patent Publication No. 249327/2003 (Tokukai 2003-249327) (published on Sep. 5, 2003) (Patent Document 6), Japanese Unexamined Patent Publication No. 327416/2003 (Tokukai 2003-327416) (published on Nov. 19, 2003) (Patent Document 7), Japanese Unexamined Patent Publication No. 50590/2005 (Tokukai 2005-50590) (published on Feb. 24, 2005) (Patent Document 8), Japanese Unexamined Patent Publication No. 36954/2003 (Tokukai 2003-36954) (published on Feb. 7, 2003) (Patent Document 9), and Japanese Unexamined Patent Publication No. 340740/2006 (Tokukai. 2006-340740) (published on Dec. 21, 2006) (Patent Document 10) disclose ion generating devices and charging devices each employing a creeping discharge system. Such ion generating devices and charging devices include an ion generating device (creeping discharge device) that consists of a discharge electrode and an inductive electrode provided to face each other with a dielectric material therebetween and that applies a pulse waveform voltage across the two electrodes to generate ions. In such ion generating devices and charging devices, a charge receiving material is provided opposite to the inductive electrode to face the discharge electrode, and is charged with generated ions.